Where trucks have a group of two or more axles with a set of springs for each axle, they normally use some form of rocker arrangement to equalise the load carried by respective leading and trailing springs. The rocker is normally connected to the spring ends either by a pin and shackle arrangement or by the use of a boot through which the spring can slide backwards and forwards and also twist sideways. Where the boot arrangement is used it is normally referred to as a "slipper" connection and a torque arm is used to locate one end of the spring. At the other end of the spring it is normal practice to turn down the end of the lower main leaf to form a hook which is designed to catch on a keeper bolt at the bottom of the boot to prevent the end of the spring from coming out of the boot during extreme axle oscillation.
FIGS. 1 to 3 of the accompanying drawings show prior art rocker arrangements. The arrangement just described is shown in FIG. 1. This connection has become popular since it allows the spring to twist sideways (i.e. when one end of the axle is caused to move up or down relative to the other end) and requires less maintenance than the pin and shackle arrangement.
However, this type of slipper connection has certain disadvantages. Firstly, since the spring end is free to move up and down inside the boot, it tends to chatter and create a lot of noise when the vehicle is running empty and can also allow the axles to hop under braking. Secondly, because the spring camber changes under load deflection it is difficult to ensure that the hook catches on the keeper bolt under all conditions. Consequently, the spring end often comes out of the boot, or alternatively, the keeper bolt becomes jammed between the leaves of the spring.
The spring end normally rests against a contact point at an upper part of the boot and various attempts have been made to keep the spring end in constant contact with the contact point. One method is to attach the spring end to the contact point by a pliable material such as rubber or polyurethane but to date none of these connections have been strong enough to be of practical use.
In an attempt to overcome this problem, the present applicants have adopted the arrangement shown in FIGS. 2 and 3 of the prior art drawings. In this case, an extra long leaf with a hooked end is provided at the bottom of the spring and exerts a pressure on the top of the keeper bolt which pushes the main leaves of the spring up against the upper contact point (as shown in FIG. 2). This approach also has its problems. The rocker boot has to be enlarged to accommodate the extra spring leaf. Furthermore, as shown in FIG. 3, the keeper bolt tends to foul on this extra leaf on upward oscillation of the rocker. Also, the extra depth of the overall spring hinders sideways twisting of the spring unless the rocker boot is made much wider than the spring.